The present invention relates in general to filter circuits and in particular to a low-pass filter circuit having a high slew rate.
Analog data acquisition systems often simultaneously acquire waveform data from several different analog signals by sampling the signals in rapid succession, employing digitally controlled multiplexers to successively connect each analog signal to the input of an analog-to-digital (A/D) converter. The A/D converter converts the DC voltage level of each waveform into digital data of proportional magnitude for storage by the acquisition system in a random access memory. When a sampled waveform contains noise, it is desirable to filter out high frequency components in the multiplexer output with a low-pass filter circuit typically having attenuation of 60 db or more at frequencies of 60 Hz or higher, but such filters normally require up to 300 milli-seconds to slew and settle in response to a relatively large voltage difference between two successively sampled signals. The scanning rate at which the multiplexer can be switched from channel to channel is therefore typically limited by the filter slew rate to about three channels per second, a rate which is much too slow to provide adequate sampling of most AC waveforms. What is needed is a low-pass filter with a high slew rate which would permit the multiplexer to sample multiple waveforms at a much higher scanning rate.